1. Field of the Invention
This invention relates to an air bag formed by joining a facing sheet and a backing sheet each made from a thermoplastic film such as urethane or the like and having a light weight and a high mechanical strength.
2. Description of Related Art
In order to stably protect passengers in an automobile during an automobile accident, an air bag is used by attaching to a steering wheel and the like for the automobile. The air bag is usually produced by cutting a base woven cloth for the air bag crosslink-coated with silicone, rubber or the like and sewing it into a bag.
Furthermore, the air bag is provided with an opening for an inflator attached to a base plate facing a gas generator. A second opening for a vent effectively absorbs shock, and portions surrounding these openings are stitched with a reinforcing cloth made from the same material as the air bag. Recently, there have been proposed air bag cloths laminated with various synthetic resin layers in order to attain a weight reduction of the air bag, the improvement of the mechanical strength without substantially causing the aging degradation, and heat-welding such as high frequency welding or the like is carried out for joining these cloths with each other.
For example, JP-A-2-114035, JP-A-2-162134, JP-A-3-7337 and JP-A-6-199198 disclose air bags obtained by joining the air bag cloths through the heat-welding.
Among them, a typical air bag is shown in FIG. 7. As shown in FIG. 7b, the air bag 21 is formed by joining a facing fabric 22 and a backing fabric 23 at a joint part 24. Numeral 26 is a gas inlet port. Each of these fabrics is laminated with a polyester resin layer 22B through a primer coat layer 22A as shown in FIG. 7a. These facing and backing fabrics 22, 23 are heat-welded by facing the polyester resin layer 22B to the polyester resin layer 23B at the joint part 24 as shown in FIG. 7c. AS a result of such joining, it is possible that the hardness and the bending property of the joint part 24 and the adhesion strength between the fabric and the resin layer and the welded strength between the resin layers improve.
However, when air bag 21 is inflated, the facing and backing fabrics 22, 23 are separated from each other at the joint part 24 as shown in FIG. 7b and hence a large peeling force is applied between the layers in the joint part 24. Since the welded strength (peel strength) between the polyester resin layers 22B and 23B is fairly high, there is no fear of causing peeling between the polyester resin layers 22B and 23B. On the other hand, the peel strength between the fabric 22 (23) and the primer coat layer 22A (23A) is not so high and hence there is a fear of causing the peeling therebetween.
In order to prevent peeling between the synthetic resin layer and the fabric at the joint part formed by joining air bag cloths, each laminated with the synthetic resin layer to each other through the heat-welding, there was proposed an air bag in JP-A-2-162134. As shown in FIG. 8, an air bag 31 is formed by piling two base cloths 32 and 33 each covered on at least one surface with rubber or a thermoplastic resin one upon the other at their outer peripheral end portions to face these covered surfaces to each other. A reinforcing cloth 34, made from the same covering material as in the base cloth is applied to an inner peripheral face of the piled portion and then heat-welding them.
In such an air bag, the force in the peeling direction between the facing and backing base cloths 32 and 33 is received as a force in the shearing direction among the base cloths 32 and 33 and the reinforcing cloth 34 to enhance the strength at the joint face.
However, the reinforcing cloth 34 is arranged along a face substantially perpendicular to the joint face between the facing and backing base cloths 32 and 33. That is, these cloths should be joined so that the reinforcing cloth 34 is three-dimensionally arranged to the joint face which is two-dimensioned. Therefore, the heat-welding device such as a high-frequency welding device or the like becomes more complicated. Furthermore, the reinforcing cloth 34 is a cylindrical body, so that the production thereof is troublesome. Moreover, if it is intended to two-dimensionally join the reinforcing cloth 34 to the joint face, a problem of leaving wrinkles on the joint face is created.